The son of Dn and brother of Gwydion fab Dn.

Gilfaethwy fell for the beautiful Goewin, who held the post of foot-holder at the court of Math fab Mathonwy, a post that could be held only by a virgin. Learning of his brother's desire, Gwydion fab Dn used his magic arts to conjure up a quarrel between Math fab Mathonwy and Pryderi. While Math fab Mathonwy was away fighting Pryderi, Gilfaethwy and Gwydion fab Dn abducted Goewin, whom they took turns in ravishing.

When Math fab Mathonwy returned and discovered what had happened, he punished the brothers by condemning them to spend three years in animal forms, the first as male and female deer, the second as swine and the third as wolves. At the end of each year, when Gilfaethey and Gwydion fab Dn had produced an offspring, Math fab Mathonwy changed them into their next form. At the end of the three years, each having been male and female, he considered their punishment to be complete and changed them back into human form.

Gilfaethwy is the Celtic origin of Griflet.